This invention relates to a press-in contact, and more particularly to a construction of the press-fitting portion of a press-in contact to be press-fitted in a through-hole in a printed circuit board for mechanical fixation and electrical connection.
Prior art press-in contacts will be explained with reference to FIGS. 4a and 5a of sectional views respectively illustrating press-fitting portions 34 of press-in contacts and FIGS. 4b and 5b of sectional views respectively illustrating the contacts press-fitted in through-holes of printed circuit boards 22.
A press-in contact is generally made of phosphor bronze or beryllium copper having springiness for press-fitting it into the through-hole 24 of a printed circuit board.
The press-fitting portion 34 of the press-in contact 32 shown in FIG. 4a has a substantially U-shaped cross-section and is press-fitted in the through-hole 24 of the printed circuit board 22 for firmly fixing the press-in contact 32 to the printed circuit board 22 as shown in FIG. 4b.
The inner edge of the press-fitting portion 34 forms a U-shaped groove 38, while the outer periphery of the portion includes a plurality of arc parts 36 and straight parts 40 whose number is one less than that of the arc parts 36.
The curved portion of the U-shaped inner edge is concentric to the arc parts 36 of the outer periphery. The straight parts 40 and the arc parts 36 are alternately arranged so that the straight parts 40 are positioned between the arc parts 36. In the embodiment shown in FIGS. 4a and 4b, there are four arc parts 36 and three straight parts 40, all these parts having substantially equal thicknesses.
When the press-in contact 32 is inserted into the through-hole 24 of the printed circuit board 22, the press-fitting portion 34 of the contact 32 is press-fitted in the through-hole 24 with its arc parts 36. In this case, the arc parts 36 will apply stresses substantially uniformly against the printed circuit board 22 (Japanese Patent Application Publication No. H4-30,717).
In the other embodiment shown in FIG. 5a, there are four arc parts 36 and three straight parts 40 and all these parts have substantially equal thicknesses. The press-in contact shown in FIG. 5a differs from that shown in FIG. 4a in the feature of a V-shaped cross-section section whose arms open somewhat outwardly. When the press-in contact 32 is inserted into the through-hole 24 of the printed circuit board 22, the press-fitting portion 34 of the press-in contact 32 is press-fitted in the through-hole 24 of the printed circuit board 22 with the arc parts 36.
The press-in contact shown in FIG. 5a has a larger press-fitting surface area than that of the contact shown in FIG. 4a but exserts non-uniform stress on the printed circuit board 22.
With each of the above described press-in contacts, since the outer periphery of the press-fitting portion 34 consists of the arc parts 36 and the straight parts 40 arranged therebetween, stress concentration would occur only on that face of the through-hole of the printed circuit board 22 which abut against the arc parts 36.
Moreover, should thicker arc parts 36 be employed for the purpose of increasing the holding force for the contact, extreme stress concentration would occur in this case only on the parts of the through-hole abutting against the arc parts 36.
The stress concentration on the printed circuit board 22 will cause whitening around the through hole 24 of the board, resulting in lowering of dielectric strength, disconnection of circuit, and lowering of insulation resistance. Even if a circuit is disconnected only at one position, exchange of the printed circuit board itself is needed, increasing the cost.
With the press-in contact of the V-shaped cross-section whose arms are in uniform thickness, such contact has an increased rigidity in its entirety but becomes poor in flexibility. Therefore, when the press-in contact is press-fitted in the through-hole of the printed circuit board, the open ends of the V-shaped cross-section do not deform so that non-uniform stress is applied onto the through-hole, tending to cause the whitening of the board.
The word "whitening" used herein means a phenomenon that part of a printed circuit board becomes opaque white resulting from peeling between laminated layers or fine cracks in the board. The whitening is usually found by telescopic observation, although it can be seen by naked eyes.